Can You Use Interior Paint Outside (Why You Shouldn’t)

Most exterior paint is designed to withstand outdoor elements like sunlight, moisture, and mold, while interior paint's chemistry is meant to survive scuffs, scratches, and handprints. 

Paint meant for outdoor use has a higher concentration of organic pigments that may not be organic like the ones in interior paint.  

Interior paint is more likely to be water based paints, also called latex paint or acrylic paints. The solvent - the liquid that serves as a vehicle for the pigment - is just water, making it easier to clean than exterior paint. 

The alternative is an oil based paints formulated with solvents like alcohol, linseed oil, esters, or other natural oils. 

Oil-based exterior paint indoors  has higher volatile organic compounds (VOCs), making the fumes more harmful for a longer time. This is also a reason why exterior paint shouldn't be used inside.

Additives like anti-mold agents, agents to prevent fading, and weatherproofing agents are also put into exterior paint. Interior paint comes in a wider variety of finishes, while exterior paint is almost always a matte or flat finish. 

Finally, exterior paints or outdoor paints tend to cost more, especially the ones with special attributes. 

This is the main reason why people are so curious about use interior paint on exterior surfaces; the money savings are admittedly tempting at first, particularly if you already have leftover interior paint. 

Not so long ago, oil-based paint was the go-to for exterior painting because it binds to painted surface better and paint resistance is more to outdoor conditions. 

Water-based paints like alkyd paints have advanced to the point where you can use either. So can emulsion/latex and acrylic paints be used outside? 

Using interior paint outside is certainly possible, but it’s a bad idea. Here’s why: 

Whether it’s water- or oil-based, exterior paint has additives that prevent its color from fading and also keep weather moisture from damaging it.

Let moisture in or paint onto a wet surface, and you could get bubbles that lead to cracking and peeling.  

Exterior formulated paints have lots of synthetic additives and interior paint binders to help it withstand interior environment and avoid chalking and breaking. 

Mold and mildew can discolor paint and create a need for a new paint job all over again.

So even if you do save some money using interior paint outside, you’ll pay more on a new coat later. 

2. More Porous & Has Lack Of Resin To Bind 

Exterior paint is less porous, which is part of the reason why it’s better at handling moisture.

Oil-based interior and exterior paint is the least porous, and while it is bad to use oil-based paints on exterior surfaces like stucco and brick, they do bind better to metal and some other interior surfaces  

Still, even comparing water-based interior and exterior paints, interior paints are more porous because it makes them easier to clean, and they don't have to withstand bad weather.  

Water-based interior and exterior paint is fast becoming the go-to variety because it has fewer fumes, it doesn’t have to be disposed of in a special way, and it's easier to use.

But water-based interior paints or indoor paints have more rigid resins to make them more durable against scrapes and scratches, making them more fragile when applied interior paint outdoors voids. 

Sunlight is a big deal for exterior paint jobs. Some interior paints will dry so quickly that you’ll wind up being able to see the marks in the finished paint job, even if you’re using a paint sprayer that atomizes and distributes leftover paint fairly evenly. 

Interior paints may also require more coats because of the weaker binding to exterior surfaces that we already mentioned. Plus, it’s not designed to handle UV rays and temperature changes. It will fade in sunlight, meaning you’ll wind up repainting sooner.  

Although a paint sprayer will make the job easier, interior paints still creates more work for you.

It has to be scored with sandpaper to create better adhesion conditions if you want to use it as a exterior primer, and you'll have to put a final coat of exterior paint inside or some kind of sealant if you want it to last. 

Some paint products come with extended or lifetime warranties that protect against defaults in the paint itself. However, if you don't follow manufacturer recommendations, i.e., using interior or exterior paint walls, then they won’t honor the warranty.  

While the warranty is easy to back out of if the paint does last for years, there’s no reason to give the manufacturer an even easier reason to squirrel out of the deal.

Stick to expensive exterior paints for external surfaces just in case there’s a defect with the paint. Even if it doesn’t come with a warranty, the paint job will be better if you use the right product. 

Technically, learning how to make exterior paint is kind of the same as learning how to make interior paint into exterior paints. 

By that, we mean you would have to procure additives to make interior paints waterproof, mold-resistant, and fade-resistant and then learn how to mix it to make it into exterior paints of the same quality as the manufacturer. 

You shouldn't mix interior paint with exterior paints because you'd be diluting the exterior paints differ and reducing its durability and waterproofing. 

Plus, unless you have exactly the same shade of paint, you could be corrupting your color by mixing two different varieties. 

The best thing to do if you’ve already painted the outside of your house with interior paint is to repaint it or cover it with high-quality exterior paint.

Hopefully, you’ll realize the mistake before you’ve made it far on the outdoor paint projects because you need to sand away some of the interior or exterior paint or prime over it to make sure you get a nice final coat with the exterior paint inside. 

If you’ve used water based paints on wood, it may swell. Any interior paint is likely to fade, crack, or chalk on an exterior surface, which will hurt the value of the house until you rectify the mistake. Better to just bite the bullet and repaint it while you’re in DIY mode.  

Try to avoid using leftover interior paint on exterior surfaces. It might seem like a good idea if you have lots of paint left over from remodeling the inside of your house, but you'll regret it eventually. 

Interior paint isn't made to withstand sunlight, rain, or mold, so it's best not to use it outside.